(1) Field of the Invention
This invention relates to ice displacement to facilitate winter navigation, particularly of elongated lake ships.
(2) Description of Prior Art
The desirability of navigation in northern climates, during as much of the year as possible, is accentuated by the tremendous capital cost of building large lake ships of the type customarily employed on the Great Lakes, and the energy shortage making ground transportation more expensive than ever.
U.S. Pat. No. 3,791,328 Schirtzinger (1974) mentions the revival of interest in expedients for cutting ice to permit the passage of vessels through icebound areas and describes means for making an ice removal track. The patent also talks about prior art expedients disclosed in U.S. Pat. Nos. 3,521,529 Rosner et al (1970) and U.S. Pat. No. 3,636,904 Blanchet (1972). These expedients would not be practical for general application to conventional vessels over normal cargo routes which have to conform to rigid standards, since this rules out radical changes in construction.
The progress of a conventional vessel through ice is dependent mainly on the thickness and type of ice; the thrust of the propeller or propellers; the shape of the hull, with particular emphasis on the forward section; and friction between the hull of the vessel and the ice. Should any of the above factors change or be changed then the vessel's performance would change. The ability of a vessel to steer, when operating in ice, is dependent principally on the thickness and characteristics of the ice; the shape of the bow section; the shape of the stern section; the thrust of the propeller and size of the rudder; and possibly of greatest influence, the length of parallel body (straight ship sides). So, although most vessels, other than ice breakers, have trouble in navigating in ice covered waterways, some have a lot more trouble than others, and this difficulty is proportionately increased with ship length.
The influence of a ship's side length on the ability of a ship to navigate in ice is discussed, in some detail, in Influence of Major Characteristics of Icebreaker Hulls on their Powering Requirements and Manoeuverability in Ice by R. Y. Edwards, Jr., R. A. Major, J. K. Kim, J. G. German, J. W. Lewis, and D. R. Miller, appearing in the Journal of the Society of Naval Architecture Marine Engineers No. 12, 1976. This is a paper presented at the Annual Meeting of the Society, in New York, on Nov. 11 to 13, 1976, which is hereby incorporated by reference. The tests of two models are discussed which differed only in the length of the parallel middle body. One model was 20% longer than the other. The predicted turning radius per unit ship length of each of the two models was plotted versus rudder angle, for an approach speed of 6 knots. The turning radius of the longer ship was significantly greater than that of the shorter model. The turning radius per unit of ship length was 10.4 for the longer model and 3.0 for the shorter model at a rudder angle of 35.degree.. An increase of 20% in ship length caused an increase in turning radius, per ship length, of 300%. The absolute turning radius was increased by 366% as compared with that of the shorter model. Hence, the tests predict much higher forces and moments for the longer ship to achieve the same radius turn and, conversely, the ship will only be able to execute a much greater radius turn with the same available turning forces.
The problem of length versus ability to turn in ice is specially acute with ships of the type now used on the Great Lakes and its affect on their difficulties is discussed in the publication Survey Study for Great Lakes--St. Lawrence Seaway Navigation Season Extension, Main Report, January 1978, U.S. Army Engineer District, Detroit Corps of Engineers, Detroit, Mich., which is hereby incorporated by reference. The lake ship is designed to maximize the cargo-carrying capacity within the restrictions of the lock sizes within the system. Essentially, this calls for a design incorporating a full bow, long parallel ship sides, and a relatively full stern. Such ships have a length of 600 to 1000 feet and a beam from 56 to 105 feet. This general standard design of lake vessel, although admirably suited for the efficient carriage of bulk cargo throughout the "lakes system" under normal navigating conditions, is not well suited to the conditions encountered in winter on the Great Lakes system. The long parallel body, making it difficult to alter course, when navigating in ice, often prevents a vessel from attempting to navigate in confined water, even though the bow is capable, although sometimes with difficulty, of breaking a channel to enable straightline forward progress to be made.
When the ship is moving straight ahead, the ice is broken by its bow and the unbroken ice tends to hug the sides and develop considerable friction, impeding forward movement. The condition is complicated by the fact that the ice is often "uneven", as a result of channels having been broken and rebroken, with the pieces of ice thrown up into uneven mounds and refreezing in that form. This uneven structure increases the friction and resistance to movement. If straightline movement is difficult, the problem is compounded when the ship tries to turn. In making a turn, under the action of the rudder, the ship pivots about a point about a third of the way from the bow to the stern (this will vary somewhat depending on the design of the vessel and its draft forward as against its draft aft). Bow thrusters are sometimes used to move the bow laterally, but these tend to become fouled in ice and so are not usually employed for winter navigation.